Method for drying carbon black pellets

ABSTRACT

Wet pellets of carbon black are dried to produce dry pellets by means of a two-step process. In the first step a bed of wet pellets is fluidized by means of a heated gas and the pellets are thus partially dried. In the second step the partially dried pellets from the first step are subjected to additional drying while in a nonfluidized state, e.g. pellets from the fluidized bed are heated and subjected to a mild, mechanically produced tumbling action during the second step.

O United States Patent 11 1 1111 3,7 Myers June 26, 1973 METHOD FORDRYING CARBON BLACK 3,238,634 3/1966 Goins 34/10 PELLETS 3,282,57711/1966 Cottle 34/57 R X 3,469,323 9/1969 Jongbloed ct 111.... 34/57 R XInventor: Claude y El Dorado. 3,513,560 5/1970 Lamare 34/10 [73]Assignee: Cities Service Company, New York,

Primary Examiner-William F. ODea Assistant Examiner-William C. Anderson[22] Flled: 1 17, 1970 Attorney-J. Richard Geamam [21] Appl. No.: 47,085

[57] ABSTRACT 521 US. 01. 34/10, 34/57 A, 34/57 R Wet pellets of carbonblack are dried to Produce y 51 1m. (:1. F26b 3/08 Pellets y means of ap Process In the first step [58] Field of Search 34/17, 10, 68, 57 D, abed of Wet Pellets is fluidized y means of a heated 34/57 R, 133, 132,108, 130, 135, 142, 60, 69, gas and the pellets are thus partiallydried. In the sec- 90; 263/32 R, 21 A 0nd step the partially driedpellets from the first step I I are subjected to additional drying whilein a nonfluid- 5 References Cited ized state, e.g. pellets from thefluidized bed are heated UNITED S E PATENTS and subjected to a mild,mechanically produced tumbling action during the second step. 245,9808/1881 Smith 34/133 2,117,822 5/1938 Pehrson et a]. 34/10 12 Claims, 1Drawing Figure CARBON BLACK POWDER SEPARATOR METERER H2O D i 304 3 v 2k28 T FEED SCREW 1 I I 7 e 22 24 l l 1 4 PELLETIZER ML h 9Q V 6 1 l FLUIDs50 DRYER\\ ROTARY DRUM I2 .4 DRYER r 1 i h 1 2e 23 AIR BLOWER FUELSUPPL Y DRY PELLETS" METHOD FOR DRYING CARBON BLACK PELLETS BACKGROUNDOF THE INVENTION Freshly manufactured carbon black powder is pelletizedto increase the bulk density thereof, thus making it easier to handleand more economical to ship. Wet pelletizing" processes are mostcommonly employed wherein an aqueous pelletizing medium and the carbonblack powder are mixed together and tumbled to form wet pellets whichcontain 35-60 weight percent of water. The pellets must then be dried toprovide a dry, pelletized product which contains less than about weightpercent of water, and more commonly less than one weight percentthereof.

The conventional method for removing water from wet carbon black pelletsis by means of an indirectly fired rotary drum dryer, since it handlesthe pellets very gently and is mechanically dependable. However, one ofthe problems associated with such dryers is the difficulty inmaintaining control of the drying operation as the throughput of wetpellets is increased beyond certain limits, for' in such cases itbecomes proportionately harder to maintain a uniform temperature profilethroughout the drier, or to maintain temperatures within the necessarylimits, commonly 325450F. As a consequence the pellet drying process canbecome a bottleneck when it is otherwise possible to increase theproduction rate in the carbon black manufacturing operation.

Efforts have been made to increase the rate at which carbon blackpellets can be dried by employing fluidized bed driers since theyprovide high drying capacities and efficiencies relative to their size.They are also smaller physically than rotary drum dryers of equalcapacity. Such efforts have been largely unsuccessful, however, sincefluidized bed processes are inclined to damage the friable carbon blackpellets, i.e. because of the extreme mobility of the pellets within thebed, they can be quickly broken up by repeatedly colliding with oneanother.

SUMMARY or THE INVENTION I It is therefore an object of the presentinvention to increase the rate at which wet pellets of carbon black canbe dried while,at the same time, minimizing damage to the pellets. Otherobjects and-advantages of the invention will become apparent from thefollowing description and the appended claims.

In accordance with the present invention, wet carbon black pellets aredried by means of a two step process, the first of which amountsto'partial drying of the pellets in a fluidized bed, this being followedby a subsequent step wherein the pellets are further subjected to dryingconditions while in a nonfluidized state. By means of this procedure,the advantages of high efficiency and high capacity which are inherentin a fluid bed process are largely retained without excessive damage tothe pellets, for it has been discovered that most of the damage whichresults from drying the pellets in a fluid bed does not occur until asubstantial portion of The amount of water which can be safely removedfrom the wet pellets by means of a fluidized bed in ac cordance with thepresent invention is subject to variation, but it should never approachsubstantially total removal of moisture, and as a general rule thepellets should be removed from the fluidized bed while they still have amoisture content of not less than about 20 weight percent.

ltis, of course, desirable to achieve as much of the drying as possibleby means of a fluid bed and the extent to which this is practical in anygiven case can be determined by experimentation. Generally, no more thanabout 10 weight percent (dry basis) of the pellets should be convertedto dust which becomes entrained in the fluidizing gas and is thus lostfrom the bed.

After partial drying in the fluidized bed, the pellets are subjected toadditional drying while in a nonfluidized state, as previouslyindicated. The term fluidized bed as used herein is intended to meanthat the wet pellets are suspended within a stream of heated gases andare thus caused to behave as a fluid, while the velocity of thefluidizing gas is maintained low enough nonetheless, to preventsubstantial entrainment and loss of the pellets from the fluidizingzone. The term nonfluidized" is thus intended to mean that the pelletsare not suspended or conveyed by means of a gas stream. Therefore, theterm nonfluidized can be applied to a static bed or to mild,mechanically-produced tumbling of the pellets in a bed such as isachieved, for instance, by means of a rotary drum drier, a rotary traydrier or a rotary rake dryer.

In accordance with the present invention the throughput of a mechanicaldryer can be greatly increased, e.g. 25-50 percent, without henderingthe ability to control the temperature of the drying operation. It willalso be appreciated that the invention can be employed to advantage inexisting operations wherein a rotary drum dryer is already beingemployed, i.e. a relatively simple and economical fluid bed drier can beinstalled ahead of the drum to considerably increase the throughput rateof the latter.

Fluidization of wet carbon black pellets in the present invention can beaccomplished by means of conventional apparatus and methods forproducing fluidized beds of particulate solids. As previously indicated,partial drying of the pellets in the first step is achieved by contactwith a heated gas which is passed through a bed of pellets to achieve astate of fluidization. After passing through the bed, themoisture-ladened gas can be conveyed to a separator for removal of anycarbon black dust therein and the cleaned gas can then be expended orutilized in some other phase of the carbon black manufacturing process.

The type of gas employed for forming the fluid bed of the pellets issubject to considerable variation, but in the ordinary case it will besubstantially nonreactive with the carbon black at the conditionsemployed for fluidization. Some gases which can be employed for thepurpose include heated air, air mixed with the hot products of burning afluid fuel with a free-oxygen containing. gas, and flue gases producedin the carbon black reactor during manufacture of the black.Satisfactory results can be obtained when the fluidizing gas has atemperature within the range of about 700F to about 1,200F whenintroduced into the bed of wet pellets. Lower or higher temperatures canbe employed, but lower temperatures will, of course, reduce the dry ingrate, and there is some danger of excessive damage to the pellets'anddetrimental chemical reaction between the black and the gas if highertemperatures are employed. When the gas is introduced into the pelletbed at a temperature within the aforesaid range, the temperature of thefluidized bed can be maintained within the range of about 130F to about160F to achieve a very high drying efficiency.

Since the resulting temperature of the fluidized bed is more or lessindependent of its height, the relatively deep bed normally used influidized bed drying operations is not required in the presentinvention. A bed depth of 6 to 12 inches can be employed to advantagewhile employing a bed depth to cross-sectional area ratio within therange of about 1 ft./l2 sq. ft. to about I ft./24 sq. ft. The ability toemploy such a shallow fluidized bed provides the advantage of a lowerpressure requirement for the fluidizing-drying gas while also reducingattritioning of the pellets within the bed.

In accordance with this invention the residence time of the pelletswithin the first and the subsequent drying stages can be established toprovide a much shorter duration in the fluidizing bed than is utilizedfor the subsequent, nonfluidized step, thereby minimizing attritioningof the pellets while still providing a substantial increase in thedrying rate. The time of exposure to drying conditions in the first stepcan, for instance, be within the range of about 5 minutes to aboutminutes while employing an exposure time of about minutes to about 30minutes in the subsequent step. Generally, temperatures within the rangeof about 250F to about 500F are maintained during the subsequent,nonfluidized step wherein the pellets are finally dried to a moisturecontent of less than one weight percent.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a somewhatdiagrammatical, partly sectional flowsheet which depicts one embodimentof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the drawing,freshly produced carbon black powder is fed into a dense tank 1 throughline 2 and is then conveyed at a constant rate into a carbon black wetpelletizer 4 by means of a feed screw 3. Water, or an aqueouspelletizing medium, is fed into the pelletizer at a constant ratethrough line 5. The mixture of water and carbon black is tumbled withinthe pelletizer to produce wet pellets which are discharged throughconduit 6 into a fluid bed dryer, generally represented at 7. The fluidbed dryer comprises an enclosure 8 which bounds an upper chamber 9a anda lower chamber 9b separated by a perforated gas-- distribution plate10. The wet pellets which are discharged into the fluid bed dryer form abed which resides above the distribution plate. This bed of wet pelletsis fluidized and dried by forcing a heated gas through plate 10 fromchamber 9b. In the illustrated case air is forced into chamber 9b andthrough plate by means of a blower 11 and an interconnecting duct 12.Duct 12 is provided with a burner 13 to which fuel is supplied at ametered rate through line 14. Combustion of the fuel at burner 13liberates heat into the air passing through duct 12 so that the bed ofwet carbon black pellets residing above plate 10 is thus fluidized andpartially dried. As an alternative, the air which is conveyed intochamber 9b can be heated by means of a tube and shell heat exchanger,not shown.

In the illustrated operation, wet pellets continuously enter the fluidbed dryer through conduit 6 while partially dried pellets continuouslydischarge from the bed through conduit 15. The fluidized bed of pelletstherefore resides above plate 10 and below the outlet 16 of the dryer.Moisture-ladened fluidizing gas is expended from the enclosure 8 throughline 17. The gas from line 17 can be discharged into a solids separator18, e.g. a cyclone, for removal of carbon black dust, and the cleanedgas can then be expended or passed back to a carbon black collectionsystem through line 19. Carbon black dust which is removed from the gasby the separator can be passed into the feed screw 3 through line 20.

After partial removal of moisture in the fluidized bed dryer 7, the wetpellets pass into a mechanical dryer for substantially complete removalof moisture which remains therein. The mechanical dryer can be a tray ortunnel dryer; but, to advantage, a rotary drum dryer as illustrated at21, can be employed. The partially dried pellets from the fluid beddryer enter the rotary drum 22 through conduit 15. The inlet end of thedrum is provided with a hood 23 which seals the interior from theatmosphere. The greater portion of the length of the drum is enclosed bya shell 24 which guides hot gases over the exterior of the drum so thatthe contents thereof are heated indirectly. These hot gases are producedby burning fuel from line 25 at the burners 26, and are expended fromthe dryer shell through line 27. Dried carbon black pellets dischargefrom the drum 22 into hood 28 and are removed through line 29 to apackaging operation. Moisture vapor from the dryer drum is removedthrough line 30 and is thence passed to a separator for removal of anycarbon black dust therein, e.g. by means of the separator represented at18.

EXAMPLE I Using an apparatus arrangement substantially as shown in thedrawing, wet carbon black pellets were produced by means of the wetpelletizer at the rate of 5,240 lbs. per hour, the water content of thepellets being 49.3 weight percent. These wet pellets were dischargeddirectly into a fluid bed dryer of rectangular cross-section and havingdimensions of 3 ft. wide X 4 ft. deep X 8 ft. high. The air distributionplate was perforated with 1/16 inch diameter holes M1 inch centers. Thebottom of the pellet outlet was located 8 inches above the distributionplate, thus providing a fluidized bed depth of 8-10 inches. Air, heatedby means of a duct burner, was introduced into the fluidized bed at atemperature of 700F and at the rate of about 1,800 SCFM. In the fluidbed dryer, the pellets were partially dried to a moisture content of 34weight percent, thus reducing the water content by about 47 percent. Thepartially dried wet pellets were thenheated to a temperature of about450F during passage through the rotary drum (5 ft.D. X SO'fLL.) and werethus dried to a moisture content of about 0.5 weight percent.

What must be emphasized is that the capacity of the rotary dryer in thisoperation was limited to the evaporation of no more than about l-,835lbs. per hour of water. By the herein prescribed use of a fluidized beddryer in conjunction with a rotary dryer, the capacity of the dryingsystem was increased by 40 percent, i.e. to about 2,585 lbs. of waterper hour, while preserving pellet quality and maintaining control of thedrying temperatures.

During this drying operation less than weight percent (dry basis) of thepellets was entrained as dust in the effluent gas stream from the fluidbed dryer. The dust was separated by means ofa cyclone separator and wasthen returned to the pelletizer via the feed screw for reconversion towet pellets.

While the invention has been described with reference to particularconditions, materials, proportions, apparatus arrangements and the like,it will be understood that various changes and modifications can be madewhich are within the spirit and scope of the invention as set forth inthe appended claims.

Therefore, what is claimed is:

1. In a process for drying wet pellets of carbon black while employingfluidized bed drying, the method for reducing conversion of the pelletsto dust which comprises:

a. fluidizing a bed of wet carbon black pellets with a heated gas andpartially drying the fluidized pellets to a moisture content of not lessthan about weight percent by contact with the gas, and

b. further drying the partially dried pellets while they are in asubstantially nonfluidized state.

2. The method of claim 1 wherein a bed of the partially dried pellets isheated and subjected to a mild, mechanically-produced tumbling actionduring said subsequent step.

3. The method of claim 1 wherein said bed of pellets is heated andtumbled by passing the partially dried pellets to and through the rotarydrum dryer.

4. The method of claim 1 wherein the gas by means of which the bed ofwet pellets is fluidized is introduced into said bed at a temperature ofwithin the range of about 500F to about 1,200F.

5. The method of claim 4 wherein the temperature of the fluidized bed ismaintained within the range of about F to about F.

6. The method of claim 1 wherein the fluidizing gas IS an.

7. The method of claim 1 wherein the fluidizing gas is air in mixturewith hot combustion products produced by burning a fluid fuel with afree-oxygen containing gas.

8. The method of claim 1 wherein the ratio of height to area of thefluidized bed is within the range of about l ft./l2 sq. ft. to about 1ft./24 sq. ft.

9. The method of claim 1 wherein the length of time to which the pelletsare exposed to drying conditions in the first step is substantiallyshorter than in the subsequent step.

10. The method of claim 9 wherein the time of exposure in the first stepis within the range of about 5 minutes to about 10 minutes and the timeof exposure in the subsequent step is within the range of about 20minutes to about 30 minutes.

11. The method of claim 1 wherein the pellets are heated to atemperature within the range of about 250F to about 500F during saidsubsequent step.

12. The method of claim 1 wherein the depth of the fluidized bed of wetpellets is maintained within the range of about 6 to about 12 inches.

2. The method of claim 1 wherein a bed of the partially dried pellets is heated and subjected to a mild, mechanically-produced tumbling action during said subsequent step.
 3. The method of claim 1 wherein said bed of pellets is heated and tumbled by passing the partially dried pellets to and through the rotary drum dryer.
 4. The method of claim 1 wherein the gas by means of which the bed of wet pellets is fluidized is introduced into said bed at a temperature of within the range of about 500*F to about 1,200*F.
 5. The method of claim 4 wherein the temperature of the fluidized bed is maintained within the range of about 130*F to about 160*F.
 6. The method of claim 1 wherein the fluidizing gas is air.
 7. The method of claim 1 wherein the fluidizing gas is air in mixture with hot combustion products produced by burning a fluid fuel with a free-oxygen containing gas.
 8. The method of claim 1 wherein the ratio of height to area of the fluidized bed is within the range of about 1 ft./12 sq. ft. to about 1 ft./24 sq. ft.
 9. The method of claim 1 wherein the length of time to which the pellets are exposed to drying conditions in the first step is substantially shorter than in the subsequent step.
 10. The method of claim 9 wherein the time of exposure in the first step is within the range of about 5 minutes to about 10 minutes and the time of exposure in the subsequent step is within the range of about 20 minutes to about 30 minutes.
 11. The method of claim 1 wherein the pellets are heated to a temperature within the range of about 250*F to about 500*F during said subsequent step.
 12. The method of claim 1 wherein the depth of the fluidized bed of wet pellets is maintained within the range of about 6 to about 12 inches. 